WHY DO SMALL WHEELS BUMP MORE?

[531colin]

Terrible brain fog today!

Take one big and one small wheel, and roll them at the same size smooth bump.
Each wheel will rise by the same amount, the height of the bump.
At the same road speed, both wheels will reach the top of the bump in the same time.
So the upward acceleration of both wheels is the same?

And yet, big wheels roll better on poor surfaces; it’s been known since the days of horse transport and unsurfaced roads, when the Mail Coach load bearing wheels were almost as tall as a man. (Deep mud could also be encountered on the roads then.)

……monster trucks for building motorways don’t have roller skate wheels, and if memory serves, your roller skate wheels could be stopped completely by a small stone.

Take one big and one small completely rigid wheel and tyre, and roll them at a completely rigid Lego brick.
By the time the tyres are on top of the brick, the big wheel will have rolled (a bit) further forward than the small wheel, so the small wheel upward acceleration is faster?
And pneumatic bicycle tyres won’t cancel this out?

......is that it? ...it feels like more of a difference than that.

[oaklec]

I think it's to do with the "angle of attack" or whatever the real term is. Bigger wheels have less.

[LWaB]

Smaller wheels start lifting later and drop sooner than a big wheel does. That means the vertical acceleration for a small wheel is greater. A 1 millimetre diameter wheel would just about exactly track the shape of the bump. A 1 metre diameter wheel would start lifting much earlier and dropping much later.

Most of the time, not all of the energy lost rolling up the bump is regained going down the bump. The additional pneumatic tyre deflection of small wheels over bumps is part of the problem.

[axel_knutt]

Axle locus of the large & small wheels as they traverse the same bump:

[Sum]

axel_knutt's diagram illustrates LWaB's explanation: the larger wheel lifts sooner. Assuming both wheels are travelling at the same speed, then the larger wheel has more time to lift up a given height, meaning it rises more slowly. This makes the larger wheel feel less bumpy.

[oaklec]

Axle locus of the large & small wheels as they traverse the same bump:


Wheels.png

That's what I meant

[Nearholmer]

The time for vertical travel is part of it, but another factor to consider is how the force of initial impact with the obstruction resolves into vertical and horizontal components. For a given bump, the larger the wheel, the shallower the angle of attack, and the lower the proportion of force resolved in the horizontal direction, so the less the horizontal shock.

If you consider a wheel so small that the axle is below the top of the bump in the diagram above, then all of the force resolves horizontally, which is a very nasty shock.

We tend to think of “bumpy” as being about up and down, but it’s also about fore and aft, in the sense of longitudinal deceleration.

I think I’ve got that right ……

[oaklec]

The time for vertical travel is part of it, but another factor to consider is how the force of initial impact with the obstruction resolves into vertical and horizontal components. For a given bump, the larger the wheel, the shallower the angle of attack, and the lower the proportion of force resolved in the horizontal direction, so the less the horizontal shock.

If you consider a wheel so small that the axle is below the top of the bump in the diagram above, then all of the force resolves horizontally, which is a very nasty shock.

We tend to think of “bumpy” as being about up and down, but it’s also about fore and aft, in the sense of longitudinal deceleration.

I think I’ve got that right ……

That's what I meant

[Barrowman]

Interesting subject!

I think all of the theory here has merit.
Can I add the flexibility of spokes / forks.
I occasionally ride my High Wheeler ( Penny Farthing) .
When I first acquired it, I was unsure if the spokes were 'tight' enough.
I discussed this with other owners and was assured that all was well .

I have experienced a few machines ( including Trikes) and noted the flex of the forks ( or lack of same in some instances) . I assume a short fork will have nominally less flex than longer bladed ones so the 'feel' will be different.

[rogerzilla]

There's no real flexibility from the spokes, at least not unless they're so slack that the wheel is compromised. Forks make a very small difference and a vintage-style tight curl at the bottom of the blades works better than the more modern banana (or straight) style.

[Carlton green]

An interesting topic, it’s all about spring in the system and the different reaction of forces when the wheel meets an obstacle. Wheels form their own ramp and roll over obstacles but if the obstacle is large relative to the wheel then they just impact into it (ride into progressively higher steps and then a wall and you’ll see what I mean). The tyre also has an effect too, deep section tyres seem to absorb road imperfections better helping the wheel to just rollover the top of them. The rolling diameter of 700c and 27x1&1/4” tyres is virtually the same, but the 700c is a deeper section tyre and gives me a better ride.

Axel_knut’s plot shows the situation well. For the smaller wheel to climb over the step the vertical acceleration must be higher (because it climbs over a shorter distance) and the wheel will feel more force opposing forward motion. Either a lump on or hole in the road is effectively the same as a step, it’s all about the rates of acceleration due to interaction with the obstacles.

Edit. It could be that some bikes with small wheels have a smaller wheel base than bikes with larger wheels, such a difference would increase the ‘nodding action’ over uneven surfaces. As an aside I believe that early Bromptons’ had a slightly shorter wheelbase and extending it improved the ride.
Of course, additionally, when you have short chain-stays your backside is more over the point of wheel to road contact so you’ll feel the impact more.
Edit. Small wheel bikes sometimes have additional suspension; as Raleigh discovered with their RSW there’s a limit to what small fat tyres can handle without introducing other problems like excessive rolling resistance. Moulton is a typical example of a bike with front and rear suspension. Bromptons’ have some suspension on the rear wheel - being sat above the rear wheel it (that wheel) carries the bulk of the riders weight.
Within limits fatter tyres do help with shock absorption - though unfortunately they also tend to have higher rolling resistance - but, as demonstrated by the Moulton, suspension works better.
Edit. Sprung saddles might also help, and particularly so if wider tyres are not available (Dawes Kingpin and Raleigh Twenty).

[simonineaston]

Axle locus of the large & small wheels as they traverse the same bump:
Wheels.png

Nice… loves a chart, me
ps the paper Doc Moulton put out once he’d finished measuring the effect of wheel size and tyre pressures makes for interesting reading, however I don't recall where I found it, I’m sorry to say!

[Nearholmer]

I think this is correct:

The resultant force is perpendicular to the tangent that defines the angle of attack, so as the angle of attack becomes greater, the horizontal component increases, and the vertical component decreases.

[plancashire]

All this was for a single bump. In addition: a larger wheel will span small dips and not drop into them as much as a smaller wheel. Roads are composed almost entirely of such surfaces. Perhaps axel-knutt could do us a nice diagram of the track of the axles over a sine wave a few wavelengths long? I'd guess it would be a more (large wheel) or less (small wheel) attenuated sine wave.

[Nearholmer]

It’s also why cycling off-road is usually a good deal more tiring, and slower, than cycling on-road, because all the energy dissipated in bumping over rough ground has to come from somewhere, that being the cyclist.

I find one of the most tiring surfaces to ride over is the typical field-margin bridleways, because they of consist of bumpy tussocks, or a series of deep cuts across the way left by tractor tyres, always just the right depth to “grab” the wheels of a bike. Sometimes, you can get some speed up by attempting to bounce off the tops of the ridges, but it doesn’t take long to get shaken off the bike if you try that!